In timing transmissions of the kind in which a chain is used to transmits power from a crankshaft to one or more camshafts of an engine, ratchet tensioners have been widely used for maintaining proper tension in the slack side of the timing chain. The tensioner typically presses against a pivoted tensioner lever, on which the chain slides, and the pivoted lever, in turn, presses against the chain, suppressing vibration.
A conventional ratchet type tensioner 500, as shown in FIGS. 6-8, comprises a hollow plunger 520, slidable in a plunger-accommodating hole 511 of a tensioner housing 510. The plunger protrudes from the housing, and is spring-biased in the protruding direction. Oil is introduced under pressure into a high pressure oil chamber formed by the housing and the hollow plunger through a check valve. Excessive tension in the transmission chain causes a force to be applied to the plunger, urging the plunger in the retracting direction. The force is absorbed by the flow of oil though the small clearance between the exterior of the plunger and the cylindrical wall of the plunger-accommodating hole in the housing.
Rack teeth 522 are formed on the exterior of the plunger, and cooperate with a pawl 540, which is pivotally mounted about a shaft 512 on the housing. The pawl has teeth 541 and 542, which are engageable with the rack teeth on the plunger, and is biased, by a spring 550, so that it rotates about shaft 512 in a direction such that its teeth are urged into engagement with the rack teeth on the plunger. The ratchet mechanism allows protruding displacement of the plunger 520, but limits retracting movement to a distance corresponding to the backlash of the ratchet mechanism. Thus, in the operation of the tensioner, if excessive tension occurs in the chain, the plunger will be pushed in its retracting direction through a distance limited by the ratchet backlash, and the force applied to the plunger is absorbed by leakage of oil through the clearance between the plunger and the wall of the plunger-accommodating hole. As the chain elongates over time, due to mechanical wear, the ratchet allows the plunger to protrude, thereby taking up the slack resulting from elongation of the chain, and maintaining proper chain tension over a long time. A typical ratchet tensioner of the type described above, and shown in FIGS. 6-9, is described more fully in U.S. Pat. No. 6,059,678, the disclosure of which is also hereby incorporated by reference.
In the conventional ratchet tensioner 500, as shown in FIG. 7, the ratchet-biasing spring 550 is accommodated in a hole 513 formed in the housing, and protrudes toward the pawl, directly engaging the pawl 540, and biasing the pawl so that its teeth are brought into contact with the rack teeth on the plunger. Where an engine is designed for high load, or high speed operation, the plunger is required to have a relatively large stroke, and the pitch of the rack teeth is correspondingly large. A consequence of the larger stroke requirement in the case of an engine designed for high load or high speed operation, is that the space between the opening of the hole 513 and the pawl must be made larger in order to provide room for expansion and contraction of the pawl-biasing spring 550. However, if the spacing between the opening of hole 513 and the pawl 540 is made too large in relation to the length of the spring 550, it is possible for the spring 550 to drop out of the spring-accommodating hole 513, as depicted in FIG. 7.
On the other hand, even if the plunger-biasing spring 550 is made sufficiently long to prevent it from dropping out of hole 513, the protruding portion of the spring can buckle or become entangled within the space X between the opening of hole 513 and the pawl 540, as shown in FIG. 8. Repeatedly buckling of the spring compromises its elasticity, and can result in failure of the ratchet mechanism.
Furthermore, since in the conventional ratchet tensioner 500, the ratchet-biasing spring 550 contacts the pawl 540 directly, the spring-contacting surface of the pawl should be flat, and should be of size corresponding to the spring diameter. Moreover, it is necessary to ensure that the radial dimensions of the parts of the pawl in the vicinity of its pivoting shaft do not become so small as to weaken the pawl excessively. As a result, the spring imposes minimum size requirements both on the width of the pawl, and on the length of the portion of the pawl contacted by the spring. These minimum size requirements present obstacles to miniaturization of the tensioner.